WB Yeats arrives in New York in 1932 for the American premiere of The Words Upon the Window Pane. Photograph: Bettmann/Corbis

This early poem by WB Yeats comes from his second collection, The Rose (1893). Superficially, it may look like a typical, heady-scented 1890s love-poem, but "The Sorrow of Love" is actually a challenge to fashionable conventions. Its bold reach and simplicity anticipate Yeats's mature style. While rich in symbolism, it has a persuasively realist grain.

There's the first line, for instance. What a stroke of genius – to begin the artistic ascent with a modest, domestic sparrow. Few words could better convey the little bird's noisy activities than "brawling", with its suggestion of territorial and sexual combat. The line might intentionally reference John Donne's "Epithalamion" and "the sparrow that neglects his life for love," but it remains a true depiction of ordinary bird behaviour. That draughtsman's gift of exact, unfussy observation would be fully developed in such later works as "The Wild Swans at Coole".

The second line leads the eye farther upwards and onwards. But, however archetypal the images of the moon and starry sky, we're still within the bounds of natural observation. While "brawling" appeals to the ear as well as the eye, the impact of the new line, thanks to the beautifully contrasted epithets "brilliant" and "milky," is luminously visual.

Yeats now signals that mere description was not his goal, and in the fourth line he passes judgment on his own, increasingly splendid list. It seems that the sparrow, the moon, the milky sky and "all that famous harmony of leaves", placed in such knowing juxtaposition, have overwhelmed human experience. "Harmony of leaves" suggests laurels and lyres. A god may be inferred – Apollo, perhaps, the supreme musician. "Blotted out", applied both to "man's image and his cry", is a phrase that could be associated with pens and writing. Is the young poet who wants to create a unique new voice for Ireland hinting that he is oppressed by the power of classical stories and symbols? Possibly, but I think it more likely that this is intended as a critique of shallowly cosmetic 1890s aestheticism.

Yeats was already mining Irish myth and folklore. The Rose includes "Fergus and the Druid", "Cuchulain's Fight with the Sea", "The Man Who Dreamed of Faeryland" and the glorious "To Ireland in the Coming Times", the latter containing the poet's solemn avocation: "Know that I would accounted be / True brother of a company / That sang, to sweeten Ireland's wrong/ Ballad and story, rann and song." At the same time, the classical tradition was embedded in his imagination and would bear important fruit. Here, in the second stanza, Yeats squares up with grand self-confidence to both Irish and classical myth-making.

"A girl arose" – the trope is that of an ancient storyteller. Of course, there is also an actual girl in Yeats's autobiographical picture at this time: Maud Gonne, who will later be compared to Helen of Troy. But the figure here is more than human. She belongs to the aisling genre, and, with those "red mournful lips" evoking the symbolic "rose" which for Yeats has erotic, mystical and nationalistic connotations, she is both the idealised beloved and the vision of Ireland.

What but Ireland itself could embody "the greatness of the world in tears"? This image conveys nationhood as simultaneously magnified and tragically "blotted out". If, by itself, the phrase seems a shade overblown, its audacity is affirmed by the two subsequent comparisons, in which Odysseus, the heroic Greek wanderer, and Priam, the defeated Trojan King, are fused in this strange, mythic-human woman with the sensuous mouth. It seems significant that these are male heroes, a reminder that Maud Gonne's political activism challenged feminine stereotype – and often disturbed her poet-lover.

And now Yeats performs a syntactic miracle. Instead of closing the second stanza, he pauses on a semi-colon and repeats the main verb, "arose", at the start of the third, to carry on an extended, sinewy, almost Miltonic sentence. The woman strides on, asserting her power, although in a devastated setting in which she seems an agent of despair.

The rhyme-words from the first stanza recur in the last, emphasising the change of tone. The eaves are still "clamorous," but the moon is "climbing upon an empty sky" (my italics). "Clamorous" and "climbing" seem to intensify the upwards-striving movement; in fact, the near-homonym, "clambering," is additionally suggested by "clamorous". The same powerful epithet, creating a similar combination of sound and movement, will recur in "The Wild Swans at Coole" when the birds "All suddenly mount / And scatter wheeling in great broken rings / Upon their clamorous wings."

At the end of "The Sorrow of Love", the man and his cry are re-framed. No longer obliterated, they are "composed", in the pictorial sense of being held together, and perhaps somewhat pacified. Painful experience has redeemed shallow aestheticism. "The Sorrow of Love" proclaims that the young poet has found one of his major themes, and begun the transformation of failed relationship into imaginative triumph.

The Sorrow of Love

The brawling of a sparrow in the eaves,The brilliant moon and all the milky sky,And all that famous harmony of leaves,Had blotted out man's image and his cry.

A girl arose that had red mournful lipsAnd seemed the greatness of the world in tears,Doomed like Odysseus and the labouring shipsAnd proud as Priam murdered with his peers;

Arose, and on the instant clamorous eaves,A climbing moon upon an empty sky,And all that lamentation of the leaves,Could but compose man's image and his cry.